Artificial neural network prediction of performance and emissions of a diesel engine fueled with palm biodiesel

Increasing of energy consumption, depletion of petroleum fuels and harmful emissions have triggered the interest to find substitute fuels for diesel engines. Palm ethyl ester was synthesized from palm oil through transesterification process. The physicochemical properties of palm biodiesel have been measured and confirmed in accordance with ASTM standards. The aim of the paper is to show the effect of different diesel-palm biodiesel blends on performance, combustion and emissions in diesel engine at engine load variation. Artificial Neural Network was used for the prediction of engine performance, exhaust emission and combustion characteristics parameters. Palm ethyl ester and diesel oil were blended in 5, 10, 15 and 20 by volume percentage. The maximum decreases in thermal efficiency, fuel–air equivalence ratio for B20 were 1.5, 3.5, 6 and 8% but the maximum increases in BSFC, exhaust gas temperature and NOx emission for B20 at full load about diesel fuel were 9, 8 and 10%, respectively. The highest decreases in CO, HC and smoke emissions of B20 about diesel oil at full load were 2, 35 and 18.5% at full load, respectively. Biodiesel blend B20 achieved the maximum declines in peak HRR, cylinder temperature and combustion duration about diesel fuel. The results of ANN were compared with experimental results and showed that ANN is effective modeling method with high accuracy. Palm biodiesel blends up to 20% showed the highest enhancements in engine performance, combustion and emission reductions compared to diesel fuel.

www.nature.com/scientificreports/ with a mixture of H 2 SO 4 and ethanol was done. Palm oil was heated up to 80 °C for the moisture removal. The mixture of reactants is 1 L palm oil, 5 ml of sulfuric acid (H 2 SO 4 ) and 250 ml of ethanol. After that, Free Fatty Acid (FFA) content was checked. If free fatty acid content becomes less than 2%, transesterification process will be used as a base catalyzed reaction produces mono alkyl esters and glycerin 46 . Blend of 250 ml ethanol and 5 gm of sodium hydroxide (NaOH) were added to the esterified oil. The oil was preheated to 65-70 °C ± 5 °C at the reaction time of 60 min. The mixture of catalyst and ethanol was stirred for 30 min and poured in oil. FFA content of mixture should be less than 0.5%. After 3 h of stirring at 700 rpm, the mixture was transferred into a separating funnel and remained to settle for 12 h. The bottom layer containing glycerin and contaminants which were eliminated and the upper layer yielded a palm ethyl ester that contained alcohol and water. Biodiesel was purified by washing it from three to four times to be purified 47,48 . A rotary evaporator was used to decrease the moisture content and obtain a pure palm ethyl ester. A diesel No. 2 was used as the reference fuel. Biodiesel blends of palm biodiesel and diesel in various volumetric proportions of 5, 10, 15, and 20% such as B5, B10, B15, and B20. Photos of biodiesel blends samples were shown in Fig. 1.
Characterization of biodiesel from palm oil. Properties of crude diesel, palm ethyl ester and palm oil are shown in Table 1. Density of palm oil and biodiesel were greater than diesel oil. Palm biodiesel flash point was higher in comparison to diesel fuel and is safe to handle. Palm oil has higher kinematic viscosity and density than diesel fuel. Palm oil viscosity was reduced by biodiesel production process. Palm biodiesel viscosity is 4.5 cst at 40 °C. Viscosity values for diesel oil, B100 and biodiesel blends were within American Society of Testing and Materials (ASTM) standards. Palm oil has lower calorific value about diesel fuel but biodiesel value was near to crude diesel. Diesel fuel has lower cetane number than biodiesel, so, the ignition delay is lower for ethyl ester.
There is no phase separation at low temperature operation because of the lower differences between the biodiesel blends in densities and viscosities values. The densities of B5, B10, B15 and B20 are 845.5, 846.5, 847 and 848 kg/ m 3 . The viscosities values were 31, 27, 20 and 13.8 cSt 49 . The retention indices of the separated fatty acids methyl esters components were calculated using fatty acids methyl esters standards (C4-C22), (Sigma Aldrich Co.) as references. There are three main types of fatty acids that can be presented in a triglyceride which is saturated (Cn: 0), monounsaturated (Cn: 1) and polyunsaturated with two or three double bonds (Cn: 2, 3). The major fatty acids in palm biodiesel were oleic, linoleic, palmitic and the stearic fatty acid. GC-MS analysis shows constituents as Myristic (1.06%), Palmitic (39.2%), Oleic (43.4%), Linoleic (10.1%) and Stearic (4.8%) (see Table 2).

Experimental setup
Experiments were carried out with a diesel engine producing 5.775 kW at 1500 rpm. Used engine is single cylinder, four stroke of DEUTZ F1L511 type (Bore = 100 mm and Stroke = 105 mm and compression ratio = 17.5). Schematic diagram of test rig is shown in Fig. 2. AC generator of maximum output power of 10.5 kW was connected to the test engine to determine the engine output power. A sharp edged orifice mounted was used to evaluate the intake airflow rate. The pressure drop across the orifice was evaluated using U tube manometer. Thermocouples of type K measured the intake air and exhaust gas temperatures. A speed tachometer recorded the engine speed. Fuel consumption was measured using the consumption time of a given fuel volume of 20 ml.  www.nature.com/scientificreports/ To estimate the cylinder pressure, a water-cooled Kistler piezoelectric pressure transducer type 601A with sensitivity of 16.5 pc/bar was employed. Cylinder pressure transducer was supported with Nexus charge amplifier of type 2692-A-0S4. To reduce the pressure signal lag and resonance, the pressure transducer was flush installed. A proximity switch identified the piston's top dead centre (TDC) (Type LM12-3004PA). The cylinder pressure data was averaged over 120 engine cycles. The data was collected using the LABVIEW program and data acquisition card (NI-USB-6210). For exhaust and smoke emissions, the MRU DELTA 1600-V gas analyzer and the OPA 100 smoke meter were utilized, respectively. The engine output power was varied from zero to 100% at a rated speed of 1500 rpm during the tests. Figure 1 depicts the experimental setup schematic diagram.
Uncertainty analysis. All devices and sensors were calibrated. The uncertainties in (HC, NO x , smoke and CO emissions) and thermal efficiency were ± 1 ppm, ± 1 ppm, ± 1.5%, ± 0.01% and ± 1.5%, respectively. Output power, T exh , BSFC and engine speed showed the maximum uncertainties as 0.85%, 0.2%, 0.15% and 2.2%, respectively. The maximum uncertainties in cylinder pressure and TDC marking are 0.2 and 1%, respectively. where: uT exh , u bp , u sfc , u N, u ther , u CO , u HC , u NOx , u pcy , u TDC are the exhaust gas temperature, Specific fuel consumption, engine speed, thermal efficiency, CO, HC, NO x emissions, cylinder pressure and Top Dead Centre (TDC) www.nature.com/scientificreports/ proximity switch uncertainties, respectively. The total uncertainty for the whole experiment is obtained to be ± 3.3%.

Results and discussion
Brake thermal efficiency (BTE). Figure 3 indicates the effect of different fuels on BTE at different engine loads. Engine load increase produced the increases in thermal efficiencies. As the engine load increased, developed pumping work would be higher and result in efficient combustion. Brake thermal efficiencies for ethyl ester mixtures were lower than diesel oil at the load range. The decrease of biodiesel calorific value leads to the decrease of thermal efficiency about crude diesel. Poor volatility, improper air-fuel mixing and high viscosity led to the poor combustion characteristics of ethyl ester. The increased consumed fuel is required to compensate for the decreased calorific value in comparison to diesel oil. The maximum decreases of thermal efficiency for B5, B10, B15 and B20 were 1.5, 3.5, 6 and 8% related to diesel oil at engine load 100%. These findings are confirmed with literature 46,47 . Brake specific fuel consumption (BSFC). BSFC plots of tested fuels at different engine loads were indicated in Fig. 3. Variations in brake specific fuel consumption with the brake power for all fuels revealed that as engine load increased with the fuel consumption increase. Due to its higher density and lower calorific value, BSFC of ethyl ester blends was higher than diesel fuel. Because biodiesel has a larger bulk modulus, it can discharge more fuel for the same displacement of the injection pump plunger. BSFC was increased drastically with B20 biodiesel blend. Biodiesel was associated with the problems of fuel vaporization and atomization. The higher fuel consumption is caused by the biodiesel blends low volatility, inappropriate air-fuel mixing, and higher viscosity. The highest burned fuel to produce the same brake power of biodiesel fuel is shown about diesel oil. The maximum increases in BSFC for B5, B10, B15 and B20 at full load about diesel fuel were 2, 4, 7.5 and 9%, respectively. These results are agreed with the references 46,48 .
Exhaust gas temperature (EGT . ). The amount of waste heat going with exhaust gases is an indication of T exh . Figure 4 shows the effect of tested fuels on T exh. at the load variation. Its increase is associated with the load increase because of the more burned fuel required to take the additional load. At all the load range, diesel fuel has the lowest exhaust gas temperature but EGT of biodiesel mixtures showed an upward trend with palm biodiesel percentage. This is owing to biodiesel blends lower calorific value, lower heat transfer rate and higher viscosity compared to diesel oil. Biodiesel was related to the increase in the exhaust heat loss. The maximum increases in exhaust gas temperature of B5, B10, B15 and B20 about diesel fuel were 2, 3, 5 and 8%, respectively. These values are confirmed with literature 47,49 .
Air-fuel equivalence ratio (AFR). Increased engine load resulted in the drop in air-fuel ratio due to the increase in injected fuel mass. Figure 4 depicts the fuel-air equivalence ratios of ethyl ester mixtures. Due to the increased fuel consumption and lower calorific value of biodiesel, the air-fuel ratio was reduced as the ethyl ester ratio increased in comparison to diesel fuel. Improper mixing of air and fuel, lower volatility and higher viscosity of biodiesel led to the fuel consumption increase. Biodiesel blends B5, B10, B15 and B20 showed the maximum decreases in fuel-air equivalence ratio of 5, 8, 10 and 12% about diesel fuel at full load, respectively. These findings are confirmed with the references 46,47 . Carbon monoxide emission (CO). At the engine load variation, CO emissions of all fuels are indicated in www.nature.com/scientificreports/ consumption increase. Carbon monoxide emission of palm ethyl ester is less than diesel fuel due to the oxygen presence and more complete oxidation. Improved combustion and air-fuel air mixing produces the decline of CO emission about diesel oil. The highest decreases in carbon monoxide emission of B5, B10 and B20 about diesel oil at full load were 2, 4, 6.5 and 11% at full load, respectively. These results are confirmed with literature 46,51 .

Nitrogen oxides emissions (NO x )
. NO x emissions of biodiesel blends are shown as function of engine load as shown in Fig. 5. When compared to diesel fuel, fueling with biodiesel or its blends increased NOx emissions. NOx formation is produced by the higher cylinder gas temperature, lower heat transfer, and nitrogen content. Thermal NOx generation is affected by the residence time, cylinder temperature and oxygen concentration. The increased nitrogen oxide emissions were caused by the higher cetane number, shorter ignition delay, and accelerated combustion. Rise in NOx emissions is caused by the increase of cylinder temperature. At higher cylinder combustion temperatures, the dissociated nitrogen combines with the oxygen, resulting in the creation of thermal NOx. Higher adiabatic flame temperature of biodiesel leads to the higher nitrogen oxide emission. The maximum increases of NO x concentration for B5, B10 and B20 were 2.5, 5, 7.5 and 10%, respectively compared to diesel fuel at full load. These results were agreed according to Luján et al. 50 and Xue et al. 51 .

Smoke opacity emission.
Smoke opacity at different engine brake power for biodiesel blends are described in Fig. 6. For the entire load range, biodiesel blended fuels produced the less smoke than diesel oil. Because of the advanced combustion and higher cetane number, smoke opacity decreased at medium and higher engine loads as the biodiesel percentage increased. The presence of oxygen and reduced rich fuel zone of palm biodiesel lead to the combustion enhancement and smoke emission reduction. The maximum reductions of smoke opacity for B5, B10, B15 and B20 were 5, 8, 13 and 18.5% compared to diesel fuel at 100% of engine load, respectively. These results were agreed with the references 52,53 .
Cylinder pressure. Cylinder pressures of diesel and ethyl ester blends at different crank angles and full load are summarized in Fig. 7. Peak cylinder gas pressure of biodiesel mixtures were lower in comparison to diesel fuel and it declined with the increase of ethyl ester content in blends. Longer ignition delay of diesel oil led to combustion starts later. It increases as one moves away from TDC during the expansion stroke and is influenced  www.nature.com/scientificreports/ by the consumed fuel during the premixed burning period. Because biodiesel has a shorter ignition delay and higher cetane number burning begins earlier than diesel oil. Premixed combustion heat release and higher pressure rise rate for diesel fuel about biodiesel blends was shown. Biodiesel has a higher bulk modulus, faster sound speed in the fuel injection pipe, and it can be injected earlier. Diesel oil produced 71.22 bar at 90.5° After Top Dead Centre (ATDC). Palm biodiesel blends B5, B10, B15 and B20 showed the peak cylinder pressure of 70, 69.2, 68.1 and 67.5 bar at 89.5, 89.4, 89.3 and 90 ATDC, respectively. Peak cylinder pressure (P max ) increased with the output power increase because of the more consumed fuel at higher loads as indicated in Fig. 8. Palm ethyl ester blends record lower P max . compared to diesel oil. As the content of ethyl ester in the mixtures increased, the peak cylinder pressure decreased. Because of the shorter ignition delay and advanced injection timing, higher density, higher bulk modulus, and higher cetane number, biodiesel has lower peak cylinder pressure. Higher fuel viscosity results in increased fuel penetration, poor atomization, slow air-fuel mixing and decreased cone angle. Biodiesel higher molecular weight and lower laminar burning velocity resulted in insufficient utilization of fuel energy. The decreases in peak cylinder pressure at 4 kW are 1.7, 2.83, 4.38 and 5.22% for B5, B10, B15 and B20, respectively in comparison to diesel fuel. These findings were confirmed with the literature 54,55 .
Heat release rate (HRR). Ethyl esters heat release rate for ethyl ester blends with diesel fuel at the engine load variation were shown in Fig. 9. HRR of diesel, palm biodiesel and their blends showed the same trend. Maximum heat release rates for ethyl ester mixtures were declined than diesel fuel. Peak HRR decreased and occurred earlier for ethyl esters blends as the percentage of ethyl ester increased about diesel oil. It is ascribed to the biodiesel higher cetane number and shorter ignition compared to conventional diesel. Earlier combustion initiation of biodiesel reduces the ignition delay about diesel oil. Amount of fuel burned in the premixed combustion phase was lower for biodiesel about diesel oil but more fuel was burned in the diffusion phase. The larger bulk modulus of ethyl ester leads to the higher sound speed in the injection system and the earlier burning of diesel oil. Diesel fuel has greater rate of pressure rise and premixed combustion heat release than biodiesel mixtures. Biodiesel has lower HRR than diesel due to the lower calorific value and shorter ignition delay. The   Figure 10 showed the ignition delay profile of all fuels at engine load variation.

Ignition delay period (ID).
Higher cetane number of palm ethyl ester blends led to the shorter ignition delay compared to diesel fuel. Ignition delay period decreased for all blends as the output power increased due to the consumed fuel and cylinder temperature increases. Biodiesel blends ignition delays were lower than diesel oil and decreased with biodiesel content increase when compared to diesel oil. Advanced start of injection is because of lower compressibility and high viscosity of biodiesel. ID is affected by cetane number. Shorter ignition delay led to reduction of premixed combustion period. Higher turbulence intensity and cylinder temperature lead to the improvement of fuel-air mixing and vaporization rate. Biodiesel higher viscosity and molecular weight increase the fuel droplet size and vaporization duration. The maximum declines in ignition delay for B5, B10, B15 and B20 at full load (4 kW) were 10, 17, 25 and 30% than diesel fuel, respectively and this confirmed with literature 56 .
Cylinder temperature. Effects of biodiesel blends on cylinder temperature at different loads were shown in Fig. 11. Cylinder temperature decrease with ethyl ester percentage increase is due to the pressure cylinder decrease and its influence on the premixed combustion stage. Less biodiesel volatility and lower evaporation rate led to the cylinder temperature decrease. The diffusion combustion stage increased as the palm biodiesel percentage was increased. The molecular structure of fatty acid ethyl ester led to the lower final boiling point and faster combustion process. Because of the pressure gradient reduction, the maximum mean cylinder temperature reduced as the biodiesel concentration increased, affecting the premixed combustion stage. Peak cylinder temperatures for diesel fuel and palm biodiesel blends B5, B10, B15 and B20 are 1820, 1783, 1767, 1745 and 1720 K, respectively.  www.nature.com/scientificreports/ Combustion duration. Influence of ethyl ester mixtures on combustion duration at engine loads variations was shown in Fig. 12. It was increased as the amount of fuel injected increased, resulting in an increase in brake power. Combustion duration is shorter for biodiesel in comparison to diesel oil. As palm biodiesel percentage increased, faster combustion led to the combustion duration reduction. Molecular structure of palm ethyl esters and its lower boiling point led to the faster combustion process 56  Artificial neural network modeling. Artificial neural network (ANN) is used to predict the engine performance, emissions and combustion characteristics. Engine load and the ethyl ester concentration in the blends are two input variables (engine load, biodiesel percentage) and eleven output variables (BSFC, BTE, exhaust gas temperature, air-fuel equivalence ratio, HC, CO, NO x , smoke emissions, peak cylinder pressure, combustion duration and ignition delay). The correlation coefficient (R 2 ) of 0.99 shows the successful engine performance, emissions and characteristics prediction.
In the application of ANN, 70% of the data trials were used for training, 15% for verification, and 15% for testing. ANN was built using the feed-forward back propagation network type, Logsig transfer function, Levenberg-Marquardt (Trainlm) function, and learning function (LEARNGDM). The feed-forward back propagation network was used to describe complex problems in system modeling and identification. Table 3 and Fig. 13 show the neural network construction of the ANN model. There are three stages to ANN prediction. The first stage  www.nature.com/scientificreports/ consists of introducing the input parameters. By analyzing the validation, training, and test data, the ANN was trained with these inputs to produce the best trained ANN. Following that, ANN accuracy control was used 44 .
Design of experiments techniques, such as RSM (response surface methodology), Taguchi method, and factorial design, can be used to determine the optimum fuel blend ratios based on engine operating parameters. RSM is used to simulate nonlinear relationships between input factors and outputs or responses. For modeling linear relationships, factorial design is used, and the Taguchi method determines only the best combination of factors for the previously determined factor levels at the start of the investigation 58 . ANN is simply interconnections of many neurons which arranged in three layers. First is the input layer data, secondly the hidden layer, and finally the output layer. Figures 14, 15 and 16 indicate that the R 2 value of BSFC, BTE, exhaust gas temperature, air-fuel equivalence ratio, HC, CO, NO x , smoke emissions, peak cylinder pressure, ignition delay and combustion duration values are 0.985, 0.9784, 0.961, 0.9897, 0.9264, 0.9563, 0.9979, 0.9955, 0.9661, 0.9765 and 0.9791 respectively from the model of ANN. If the value of R 2 is close to 1 and this leads to better results from the model. The best fit is obtained with NO x emission. The best fit is shown by the highest value of R 2 . The prediction model is accurate and reliable with measured data [36][37][38][39] . The mathematical model's accuracy was determined by comparing the actual experimental results to the mathematical model output values. When the difference between the output values and the actual experiments was smaller, the accuracy increased 57,58 .
A comparative analysis between numerical and experimental data at full load condition which shows BTE, BSFC, EGT, AFR, CO, NO X , HC, Smoke, Peak cylinder Pressure, peak HRR, Ignition delay and combustion duration was shown in Table 4. This table shows the differences between experimental and numerical values to show the error.  www.nature.com/scientificreports/

Conclusions
Palm ethyl ester was produced from its oil and its properties were agreed with ASTM standards. A single cylinder C.I.E. was run with diesel and palm biodiesel mixtures (5, 10, 15 and 20%) at different engine loads. Performance, emissions and combustion characteristics were in comparison to diesel oil. The most important points are summarized as follows: • The maximum decreases in thermal efficiency, fuel-air equivalence ratio for B20 were 1.5, 3.5, 6 and 8% but the maximum increases in BSFC, exhaust gas temperature and NO x emission for B20 at full load about diesel fuel were 9, 8 and 10%, respectively. • The highest decreases in CO, HC and smoke emissions of B20 about diesel oil at full load were 2, 35 and 18.5% at full load, respectively. • Maximum decrease in peak cylinder pressure at full load is 5   www.nature.com/scientificreports/